yes, the singer should always be allowed to sing how he/she wants.
mind you the same bad manners occur occasionally in music sessions,where musicians do not pay courtesy to the person who started the tune, please listen and play how they want to play it, it is a particular problem with hornpipes,the same applies to strummers if you do not know the chord sequence, shut up, or play quietly,you strummers are alright if you know the chords, if you dont, you are a hindrance.
   however it is wonderful when you do get good chorus singing.
people should think of the voice as an instrument,no one would have the cheek to get up on stage with the main performer[uninvited with an instrument]and play along slowing the main performer down.